Doped zinc oxide powder, process for its preparation, and its use

ABSTRACT

Pyrogenically prepared, doped zinc oxide powder, wherein the doping component comprises at least one oxide from the group aluminum, gallium, indium, germanium, tin and is present in the doped zinc oxide powder in an amount of from 0.005 to 15 wt. %, and wherein the doped zinc oxide powder is in the form of aggregates of primary particles having a mean maximum diameter of from 30 to 400 nm. It is prepared by oxidation from zinc powder and at least one doping agent, wherein the process zones vaporisation, nucleation, oxidation and quenching are passed through and the doping agent is metered in in the nucleation zone, in which the temperature is below the boiling temperature of zinc. The doped zinc oxide powder can be used in electrically conductive lacquers and coatings.

The invention relates to a doped zinc oxide powder, to its preparationand to its use.

Electrically conductive materials are required for many applications,such as, for example, in plastics, lacquers, coatings or fibres. Inaddition to electrical conductivity, it is in many cases desirable forsuch materials also to be largely transparent, for example in the caseof pale or coloured coatings.

Examples of conductive materials are zinc oxides doped with oxides ofthe metals of the third or fourth main group.

It is known to prepare doped zinc oxides by coprecipitation from asolution of an alkali zincate and a doping agent by means of an acid(EP-A-404087, EP-A-405364).

It is also known to prepare doped zinc oxides by oxidation of a vapourmixture comprising zinc powder and a doping agent in anoxygen-containing atmosphere.

EP-A-598284 describes a process for the preparation of doped zinc oxideby oxidation of zinc vapour in the presence of doping agents selectedfrom the group of the chlorides and bromides of aluminium, gallium,indium, tin, germanium or silicon. The doping agents are limited so thatthey must be free of oxygen atoms and their boiling points must not behigher than that of the zinc.

The known preparation processes usually yield needle-shaped orthree-dimensionally branched, needle-shaped particles having sizes inthe micrometer range. Such particles can have advantages over sphericalparticles in terms of electrical conductivity. However, their poorsintering behaviour and their poor dispersibility are disadvantageous.

Known spherical, doped zinc oxide particles are obtained by calcinationof zinc oxide powder and gallium oxide in the presence of carbon in areducing atmosphere (JP58-145620).

Similar processes are described in JP59-097531, JP58-145620 and U.S.Pat. No. 3,538,022. The disadvantage of the spherical particles preparedby those known processes is often their lower conductivity and lowertransparency compared with the needle-shaped particles.

The object of the invention is to prepare an electrically conductivedoped zinc oxide powder which has high transparency and does not havethe disadvantages of the prior art.

The object is achieved by a pyrogenically prepared, doped zinc oxidepowder, wherein the doping component comprises at least one oxide fromthe group comprising aluminium, gallium, indium, germanium, tin,silicon, which is characterised in that the doped zinc oxide powder isin the form of aggregates having a mean maximum diameter of from 30 to400 nm and the doping component is present in an amount of from 0.005 to15 wt. %.

Pyrogenically is to be understood as meaning the formation of doped zincoxide by flame oxidation and flame hydrolysis. According to theinvention, flame oxidation is to be understood as meaning the oxidationof zinc to zinc oxide in the gas phase in a flame produced by thereaction of a combustion gas, preferably hydrogen, and oxygen. Accordingto the invention, flame hydrolysis is to be understood as meaning thehydrolysis and subsequent oxidation of the doping agents in the sameflame.

In that process, highly disperse, non-porous primary particles are firstformed, which grow together to form aggregates as the reactionprogresses, and those aggregates may join together further to formagglomerates.

Doping component is to be understood as meaning one or more oxides ofthe above-mentioned metals, as are present in the powder according tothe invention. Doping agent is to be understood as meaning a substancewhich carries an above-mentioned metal as metal component and which isconverted into the oxide during the preparation of the powder accordingto the invention. The content of doping component in the zinc oxidepowder according to the invention is based on the particular oxide inquestion.

The primary particles are to be understood as being, in high-resolutionTEM images, the smallest particles which obviously cannot be broken downfurther. Several primary particles can join together at their points ofcontact to form aggregates. Such aggregates are either difficult toseparate again by means of dispersing devices or cannot be separated atall. Several aggregates can join together loosely to form agglomerates,it being possible for that process to be reversed again by suitabledispersion.

The mean maximum aggregate diameter is determined by image analysisfollowing ASTM 3849-89. In that process, the maximum diameter of about1500 aggregates is determined and the arithmetic mean is calculatedtherefrom. The maximum mean diameter describes the structure of anaggregate more accurately than the mean diameter.

The mean maximum aggregate diameter of the zinc oxide powder accordingto the invention may preferably have a value of from 50 to 300 nm andparticularly preferably from 80 to 200 nm. Within those ranges, theelectrical conductivity and the transparency have particularlyadvantageous values.

The aggregates of the powder according to the invention preferably havea largely anistropic structure, defined by a form factor F (circle) ofless than 0.5. The parameter F (circle) describes the deviation of anaggregate from an ideal circular form. F (circle) is equal to 1 for anideal circular object. The smaller the value, the further the structureof the object from the ideal circular form. The definition of theparameter is in accordance with ASTM 3849-89.

The mean primary particle diameter of the zinc oxide powder according tothe invention, likewise determined from image analysis in accordancewith ASTM 3849-89, may advantageously be from 5 to 30 nm.

The BET surface area of the zinc oxide powder according to theinvention, determined in accordance with DIN 66131, may vary within widelimits, from 5 to 100 m²/g. Values of from 30 to 70 m²/g are preferred.

The resistivity of the zinc oxide powder may also vary over a widerange. For applications in which zinc oxide is used in the form of anelectrically conductive powder, it should be not more than 10⁶ Ohm×cm.It is preferably from 10² to 10⁴ Ohm×cm, at a compressed density of 1.0g/cm³.

In addition, the transmission of the zinc oxide powder may have a valueof more than 70%. That may be of importance for applications in whichhigh transparency is required.

The amount of doping component in the zinc oxide powder according to theinvention varies from 0.005 to 15 wt. % zinc oxide powder. The dopingmay preferably be from 0.1 to 6.0 wt. %. The range from 0.5 to 3.0 wt. %is particularly preferred.

A preferred doping component may be aluminium oxide.

A further preferred doping component may be a mixture of indium oxideand tin oxide. The amount of indium oxide is preferably from 90 to 99wt. %, based on the sum of indium oxide and tin oxide, in each casecalculated as oxide.

The invention also provides a process for the preparation of the dopedzinc oxide powder according to the invention, which process ischaracterised in that the powder is obtained in four successive zones, avaporisation zone, a nucleation zone, an oxidation zone and a quenchingzone, from zinc powder and at least one doping agent,

-   -   wherein, in the vaporisation zone, zinc powder is vaporised in a        flame of air and/or oxygen and a combustion gas, preferably        hydrogen, with the proviso that the reaction parameters are so        chosen that oxidation of the zinc does not occur,    -   and wherein, in the nucleation zone, into which there passes the        hot reaction mixture from the vaporisation zone, consisting of        zinc vapour, water vapour as the reaction product of the flame        reaction, and optionally excess combustion gas, is cooled to        temperatures of from 500 to 900° C. or is cooled by means of an        inert gas, and an aerosol containing at least one doping agent        is fed in in an amount that corresponds to the desired amount of        the doping agent in the zinc oxide powder,    -   and wherein, in the oxidation zone, the mixture from the        nucleation zone is oxidised with air and/or oxygen,    -   and wherein, in the quenching zone, the oxidation mixture is        cooled to temperatures of less than 400° C. by the addition of        cooling gas.

The aerosol may be obtained from aqueous, alcoholic andaqueous-alcoholic solutions or suspensions containing at least onedoping agent.

Where more than one doping agent is used, the aerosols may be producedand introduced into the nucleation zone together or separately.Production of the aerosols may be carried out, for example, by means ofa binary nozzle or by ultrasonic atomisation.

The process according to the invention may also be carried out bysupplying the doping agent or agents to the nucleation zone in vaporisedform instead of in the form of aerosols. In that case, the doping agentscan be vaporised in the same manner as zinc powder, that is to say in aflame of air and/or oxygen and a combustion gas, preferably hydrogen,with the proviso that the reaction parameters are so chosen thatoxidation of the doping agent does not occur. The doping agents may bevaporised separately or together with the zinc powder.

The process according to the invention can be carried out by supplyingair and/or oxygen and the combustion gas at one or more locations withinthe vaporisation zone. Likewise, air and/or oxygen and the combustiongas can be supplied at one or more locations within the oxidation zone.

Separation of the zinc oxide powder from the gas stream may be carriedout by means of filters, cyclone, washer or other suitable separators.

FIG. 1 a-b show a simplified reaction scheme. In the figures:

-   A=zinc powder, A_(v)=zinc vapour, A_(Nu)=zinc particles in the    nucleation zone,-   B=doping agent, B_(v)=vaporised doping agent, B_(Nu)=doping agent in    the nucleation zone, B_(Ae)=doping agent in aerosol form-   C=water,-   P=doped zinc oxide powder,-   a=combustion gas, b=air and/or oxygen, c=inert gas (cooling gas),-   I_(A)=zinc powder vaporisation, I_(B)=doping agent vaporisation,    I_(A+B)=vaporisation of zinc powder and doping agent together,    I_(Ae)=conversion of doping agent into aerosol-   II=nucleation, III=oxidation, IV=quenching.

FIG. 1 a shows a variant in which the zinc powder is vaporised and thedoping agent is introduced into the nucleation zone in the form of anaerosol. FIG. 1 b shows a variant in which zinc powder and the dopingagent are vaporised together.

During the vaporisation of the zinc powder and optionally also of thedoping agents, it is possible to use an excess of combustion gas,expressed in lambda values of from 0.5 to 0.99, preferably from 0.8 to0.95.

It may also be advantageous for the temperature in the nucleation zoneto be from 700° C. to 800° C.

Further variable process parameters are, for example, the rate ofcooling and the dwell time in the individual stages of the process.

The rate of cooling in the nucleation zone is preferably from 100 K/s to10,000 K/s, with values from 2000 K/s to 3000 K/s being particularlypreferred. The rate of cooling in the quenching zone is preferably from1000 K/s to 50,000 K/s, with values from 5000 K/s to 15,000 K/s beingparticularly preferred.

The dwell time in the vaporisation zone is preferably from 0.1 s to 4 s,with values from 0.5 s to 2 s being particularly preferred. The dwelltime in the nucleation zone is preferably from 0.05 s to 1.00 s,particularly preferably from 0.1 s to 0.2 s. The dwell time in theoxidation zone is preferably from 5 ms to 200 ms, with values from 10 msto 30 ms being particularly preferred. The dwell time in the quenchingzone is preferably from 0.05 s to 1.00 s, with values from 0.1 s to 0.2s being particularly preferred.

Halides, nitrates, alkyls, alkoxides and/or mixtures thereof may be usedas doping agents. Particular preference is given to the use of thehalides of aluminum, indium and tin.

The zinc oxide powder according to the invention can be used inelectrically conductive, optionally transparent lacquers and coatings,as a filler or in sun protection formulations.

The doped zinc oxide powder according to the invention acquires itsproperties, such as, for example, a defined aggregate size, electricalconductivity, or transparency, owing to the novel preparation process.Compared with the prior art, where pyrogenic processes always start withthe oxidation of the vapour of zinc and doping agent, the zinc vapour inthe process according to the invention is cooled below the boiling pointof the zinc prior to the oxidation. As a result, nucleation with theformation of zinc crystallites is able to occur. The doping agent isadded to that nucleation zone.

The mechanism of that formation and the structure of the crystalliteshas not been clarified. By varying the process parameters, such as, forexample, rates of cooling, dwell times and/or temperatures, it ispossible to adapt the properties of the powder to the particularrequirements.

EXAMPLES

Analytical Methods

The BET surface area is determined in accordance with DIN 66131.

The TEM images are obtained using a Hitachi TEM device, type H-75000-2.Approximately from 500 to 600 aggregates are evaluated by means of theCCD camera of the TEM device and subsequent image analysis.

The parameter F (shape) is equal to the quotient of the minimumaggregate diameter to the maximum aggregate diameter. The parameter F(circle) is calculated as follows: F (circle)=4Π×mean surfacearea)/2(P), where P=circumference of the aggregates.

The parameters F (shape) and F (circle) describe the deviation of aparticle from an ideal circular shape. F (shape) and F (circle) are 1for an ideal circular object. The smaller the value, the more removedthe structure of the object from the ideal circular shape. Theparameters are defined in accordance with ASTM3849-89.

Transmission

The transmission of the powders is determined in a dispersion whichcontains 1 wt. % powder and 99 wt. % water and is dispersed first bymeans of a dissolver (2000 rpm, 5 min) and then by means of anultrasonic finger (amplitude 80%, 4 min). 2.0 g of the dispersion areremoved and made up to 100 g with water. The transmission and scatteredlight for that dispersion are determined using a turbidimeter (Hach,2100AN turbidimeter).

Resistivity

The resistivity of the powders is measured at room temperature and 40%relative humidity as a function of the compressed density (0.0–1.6g/cm³). To that end, the specimen is placed between two movableelectrodes, and the current flow is determined after application of adirect current. The density of the powder is then increased stepwise byreducing the distance between the electrodes, and the resistance ismeasured again. The measurement is carried out following DIN IEC 93.

Example 1

Zinc powder (1000 g/h, particle size ≦5 μm) is transferred by means of anitrogen stream (2.5 m³/h) into a vaporisation zone in which ahydrogen/air flame (hydrogen: 4.78 m³/h, air: 10.30 m³/h, lambda=0.9) isburning. The zinc powder is vaporized thereby. The reaction mixture ofzinc vapour, hydrogen, nitrogen and water is then cooled to atemperature of 850° C. by the metering in of 2 m³/h of nitrogen, and 300g/h of a 10 wt. % aqueous aluminium chloride solution (AlCl₃×6 H₂O) arefed in in the form of an aerosol. 3 m³/h of oxidising air and 20 m³/h ofquenching gas are then added, whereupon the reaction temperature fallsto values of about 530° C. The doped zinc oxide powder is separated fromthe gas stream by filtration.

Example 2

Zinc powder (1000 g/h, particle size ≦5 μm) is transferred by means of anitrogen stream (2.5 m³/h) into a vaporisation zone in which ahydrogen/air flame (hydrogen: 4.53 m³/h, air: 9.68 m³/h, lambda=0.9) isburning. The zinc powder is vaporised thereby. The reaction mixture ofzinc vapour, hydrogen, nitrogen and water is then cooled to atemperature of 870° C. by the metering in of 2 m³/h of nitrogen, and 350g/h of a 5 wt. % aqueous indium (III) chloride solution (InCl₃×4 H₂O)are fed in in the form of an aerosol. 3 m³/h of oxidising air and 20m³/h of quenching gas are then added, whereupon the reaction temperaturefalls to values of about 680° C. The doped zinc oxide powder isseparated from the gas stream by filtration.

Example 3

Zinc powder (950 g/h, particle size ≦5 μm) is transferred by means of anitrogen stream (2.5 m³/h) into a vaporisation zone in which ahydrogen/air flame (hydrogen: 4.53 m³/h, air: 9.68 m³/h, lambda=0.9) isburning. The zinc powder is vaporised thereby. The reaction mixture ofzinc vapour, hydrogen, nitrogen and water is then cooled to atemperature of 880° C. by the metering in of 2.5 m³/h of nitrogen, and320 g/h of a 6 wt. % aqueous solution of a 95:5 mixture (based on therespective oxides) of indium (III) chloride (InCl₃×4 H₂O) and tintetrachloride (SnCl₄) are fed in in the form of an aerosol. 3 m³/h ofoxidising air and 20 m³/h of quenching gas are then added, whereupon thereaction temperature falls to values of about 650° C. The doped zincoxide powder is separated from the gas stream by filtration.

Example 4 (Comparison Example)

Zinc powder (200 g/h, particle size ≦5 μm) and 14.3 g/h of aluminiumchloride are transferred by means of a nitrogen stream (1.5 m³/h) into avaporisation zone in which a hydrogen/air flame (hydrogen: 5 m³/h, air:23 m³/h, lambda =1.93) is burning. The reaction mixture of zinc vapour,doping agent, hydrogen, nitrogen and water is then cooled to atemperature of 990° C. by the metering in of 1.5 m³/h of nitrogen. 5m³/h of oxidising air and 15 m³/h of quenching gas are then added,whereupon the reaction temperature falls to values of about 440° C. Thedoped zinc oxide powder is separated from the gas stream by filtration.

Example 5 (Comparison Example)

Zinc powder (300 g/h, particle size ≦5 μm) is transferred by means of anitrogen stream (1.5 m³/h) into a vaporisation zone in which ahydrogen/air flame (hydrogen: 4.6 m³/h, air: 9.0 m³/h, lambda=0.84) isburning. The zinc powder is vaporised thereby. The reaction mixture ofzinc vapour, hydrogen, nitrogen and water is then cooled to atemperature of 870° C. by the metering in of 1.5 m³/h of nitrogen. 4m³/h of oxidising air and 30 m³/h of quenching gas are then added,whereupon the reaction temperature falls to values of about 300° C. Thedoped zinc oxide powder is separated from the gas stream by filtration.

The process parameters for the tests are shown in Table 1, and theproduct properties of the resulting powders are shown in Table 2.

The powders of Examples 1 and 3 prepared by the process according to theinvention have a mean maximum aggregate diameter of approximately from110 to 150 nm. Very good values for the transmission and resistivity areobtained. The powder of Comparison Example 4, in which the oxidationtakes place above the boiling temperature of the zinc powder, has a meanmaximum aggregate diameter markedly higher than 300 nm. The resultingvalues for transmission and resistivity are markedly above those of thepowders according to the invention of Examples 1 to 3. Example 5describes the preparation of an undoped zinc oxide powder, which isobtained by oxidation below the boiling temperature of zinc. Thetransmission of that powder is comparable with that of the powdersaccording to the invention, but its resistivity is markedly higher.

TABLE 1 Process parameters Example 1 Example 2 Example 3 Example 4Example 5 Vaporisation Zinc g/h 1000 1000 950 200 + 14.3 ⁽¹⁾ 300Nitrogen m³/h 2.5 2.5 2.5 1.5 1.5 Hydrogen m³/h 4.78 4.53 4.53 5 4.6 Airm³/h 10.3 9.68 9.68 23 9.0 Lambda 0.9 0.9 0.9 1.93 0.84 NucleationDoping agent InCl₃ × 4H₂O InCl₃ × 4H₂O + AlCl₃ × 6H₂O — SnCl₄ ⁽²⁾ Amountof g/h 300 g/h 350 g/h 320 g/h — — doping agent (10% sol.) (5% sol.) (6%sol.) Cooling gas m³/h 2 2 2.5 1.5 1.5 Temperature ° C. 850 870 880 990870 Oxidation Oxidising air m³/h 3 3 3 5 4 Quenching Quenching gas m³/h20 20 20 15 30 Temperature ° C. ca. 530 ca. 680 ca. 650 ca. 440 ca. 296⁽¹⁾ Zinc (200 g/h) and aluminium chloride (14.3 g/h) are vaporisedtogether; ⁽²⁾ Ratio: 95:5 (based on oxides)

TABLE 2 Product properties Example 1 Example 2 Example 3 Example 4Example 5 Doping Aluminium Indium Indium oxide/ Aluminium — oxide oxidetin oxide oxide Doping amount wt. %   2.0   2.6 2.8/0.15 3.0 — Meanmaximum nm 109  145  130  1374    133    aggregate diameter Form factor  0.41   0.41   0.41   n.d. ⁽¹⁾ 0.32 F(circle) Form factor   0.64   0.63  0.62 n.d 0.61 F(shape) Mean aggregate nm² 4734  8666  7243  n.d. n.d.surface area Mean primary nm 23 20 19 n.d. n.d. particle diameter BETsurface area m²/g 20 23 25 8.3 20   Resistivity Ω × cm  10²  10⁵  10³4.3 × 10⁹ 10⁸   Transmission %  71.2  83.8  83.8 n.d. 72.7  ⁽¹⁾ n.d. =not determined

1. A pyrogenically prepared, doped zinc oxide powder, comprising adoping component, which comprises at least one oxide selected from thegroup consisting of aluminum oxides, gallium oxides, indium oxides,germanium oxides, tin oxides, and silicon oxides, and wherein the dopedzinc oxide powder is in the form of aggregates having a mean maximumdiameter of from 30 to 400 nm, and wherein the doping component ispresent in an amount from 0.005 to 15 wt. %.
 2. The zinc oxide powderaccording to claim 1, wherein the mean maximum aggregate diameter has avalue of 50 to 300 nm.
 3. The zinc oxide powder according to claim 1,wherein the aggregates have a largely anisotropic structure defined by aform factor F(circle) of less than 0.5.
 4. The zinc oxide powderaccording to claim 1, wherein the mean primary particle diameter is from5 to 30 nm.
 5. The zinc oxide powder according to claim 1, wherein theBET surface area is from 5 to 100 m²/g.
 6. The zinc oxide powderaccording to claim 1, wherein said powder has a resistivity of not morethan 10⁵ Ohm×cm.
 7. The zinc oxide powder according to claim 1, whereinsaid powder has a transmission of at least 70%.
 8. The zinc oxide powderaccording to claim 1, wherein the amount of the doping component is from0.2 to 6.0 wt. %.
 9. The zinc oxide powder according to claim 1, whereinthe doping component is aluminum oxide.
 10. The zinc oxide powderaccording to claim 1, wherein the doping component is a mixture ofindium oxide and tin oxide.
 11. A process for the preparation of zincoxide powder, the process comprising preparing, from zinc powder and atleast one doping agent, the zinc oxide powder of claim 1, wherein thepreparing is in four successive zones, which are a vaporization zone, anucleation zone, an oxidation zone and a quenching zone, and wherein, inthe vaporization zone, zinc powder is vaporized in a flame of air and/oroxygen and a combustion gas, to form a hot reaction mixture, with theproviso that the reaction parameters are so chosen that oxidation of thezinc does not occur, and wherein, in the nucleation zone, into whichthere passes the hot reaction mixture from the vaporization zone,comprising zinc vapour, water vapour as the reaction product of theflame reaction, and optionally excess combustion gas, said mixture iscooled to temperatures from 500 to 900° C. or is cooled by means of aninert gas, and wherein, at least one doping agent in vaporized form, oran aerosol containing at least one doping agent, is fed in in an amountthat corresponds to the desired amount of the doping agent in the zincoxide powder, to form a mixture, and wherein, in the oxidation zone, themixture from the nucleation zone is oxidized with air and/or oxygen, toform an oxidation mixture, and wherein, in the quenching zone, theoxidation mixture is cooled to temperatures less than 400° C., by theaddition of cooling gas.
 12. The process according to claim 11, whereinthere is fed to the nucleation zone, instead of the aerosol, the atleast one doping agent in vaporized form.
 13. The process according toclaim 11, wherein an excess of combustion gas, expressed in lambdavalues from 0.5 to 0.99, is used in the vaporization of zinc powder andthe at least one doping agent.
 14. The process according to claim 11,wherein the temperature in the nucleation zone is from 700° C. to 800°C.
 15. The process according to claim 11, wherein the rate of cooling isfrom 100 K/s to 10,000 K/s in the nucleation zone, and from 1000 K/s to50,000 K/s in the quenching zone.
 16. The process according to claim 11,wherein the dwell time is from 0.1 s to 4 s in the vaporization zone,from 0.05 s to 1.00 s in the nucleation zone, from 0.05 s to 1.00 s inthe quenching zone, and from 5 ms to 200 ms in the oxidation zone. 17.The process according to claim 11, wherein the at least one doping agentis selected from the group consisting of halides, nitrates, alkyls,alkoxides and mixtures thereof.
 18. An electrically conductive,optionally transparent lacquer or coating, comprising the zinc oxidepowder of claim 1, and one or more additives.
 19. A filler, comprisingthe zinc oxide powder of claim 1, and one or more additives.
 20. A sunprotection formulation, comprising the zinc oxide powder of claim 1, andone or more additives.